Conventionally, a capacitance type acceleration sensor formed by using a MEMS (Micro Electro Mechanical System) technology is known. Such a capacitance type acceleration sensor in general has a structure in which a proof mass (a weight: a movable portion) that is provided in the sensor and has a predetermined mass is supported by a beam or the like. As a result, when the acceleration sensor undergoes acceleration, the proof mass provided in the sensor is moved by an inertial force. Therefore, by equating the amount of movement with a change in capacitance value, the acceleration is detected.
Incidentally, as the capacitance type acceleration sensor described above, conventionally, an acceleration sensor having comb teeth on the sides of the proof mass is generally known.
FIG. 47 is a plan view showing an example of a conventional acceleration sensor. With reference to FIG. 47, in a conventional acceleration sensor 100, a proof mass 102, beam portions 103, supporting portions 104, fixed electrodes 105, and the like are formed on a silicon substrate 101 by a surface micromachining technique. In addition, the proof mass 102 has comb teeth 102a on the sides thereof. Moreover, the fixed electrodes 105 are each composed of a first fixed electrode 106 and second fixed electrodes 107, and the first fixed electrode 106 is composed of a fixed electrode supporting portion 106a and comb tooth-shaped fixed electrodes 106b formed on the side of the fixed electrode supporting portion 106a, the comb tooth-shaped fixed electrodes 106b being placed alternately with the comb teeth 102a of the proof mass 102. Furthermore, the second fixed electrodes 107 are formed so as to extend parallel to the comb tooth-shaped fixed electrodes 106ba as seen in a plan view. At least part of the second fixed electrode 107 is arranged between the comb tooth 102a of the proof mass 102 and the comb tooth-shaped fixed electrode 106b. Moreover, the proof mass 102 is generally made of polysilicon (a conductor), and is so constructed that the comb teeth 102a of the proof mass 102 function as movable electrodes 102a. As a result, the sides of the comb teeth (movable electrodes) 102a formed in the proof mass 102 and the sides of the fixed electrodes 105 (the comb tooth-shaped fixed electrodes 106b and the second fixed electrodes 107) form capacitors.
In the above-described conventional acceleration sensor 100 shown in FIG. 47, when the sensor undergoes acceleration, an inertial force acts on the proof mass 102, whereby the proof mass 102 moves in a horizontal direction (a direction indicated by arrow X or Y). This causes a change in distance between the sides of the movable electrodes 102a and the fixed electrodes 105 (the comb tooth-shaped fixed electrodes 106b and the second fixed electrodes 107), resulting in a change in capacitance value. By detecting the change in capacitance value, the acceleration the sensor undergoes is detected.
However, in the above-described conventional acceleration sensor 100 shown in FIG. 47, to increase the capacitance of the capacitors formed with the sides of the comb teeth (movable electrodes) 102a of the proof mass 102 and the sides of the fixed electrodes 105 (the comb tooth-shaped fixed electrodes 106b and the second fixed electrodes 107), it is necessary to increase the area of the sides of the comb teeth 102a and the area of the sides of the fixed electrodes 105 (the comb tooth-shaped fixed electrodes 106b and the second fixed electrodes 107). This makes it necessary to increase the thickness of the proof mass 102 and the fixed electrodes 105, resulting in the inconvenience of having to use a DRIE (deep reactive ion etching) process when the comb teeth 102a are formed in the proof mass 102a, or when the fixed electrodes 105 are formed. This undesirably reduces production efficiency.
It is for this reason that a capacitance type acceleration sensor that can prevent a reduction in production efficiency has been conventionally proposed (see, for example, Non-Patent Document 1).
FIG. 48 is a schematic sectional view showing the structure of a conventional acceleration sensor proposed in Non-Patent Document 1 described above. With reference to FIG. 48, in a conventional acceleration sensor 200 proposed in Non-Patent Document 1 described above, two electrodes (a first electrode 202 and a second electrode 203) are disposed on the upper surface of a silicon substrate 201 so as to be adjacent to each other. Here, in the acceleration sensor 200 proposed in Non-Patent Document 1 described above, as a result of a voltage being applied between electrodes, a fringe field 204 (an electric field generated beside a space between the electrodes) is generated between the first electrode 202 and the second electrode 203. In addition, above the first electrode 202 and the second electrode 203, a proof mass 205 made of parylene (relative permittivity: 3.15) is supported by beam portions 206 so as to be located in the fringe field 204. Incidentally, the proof mass 205 is so structured as to be movable in a vertical direction (a direction indicated by arrow Z) with respect to the upper surface of the silicon substrate 201.
In the conventional acceleration sensor 200 structured as described above, when the proof mass 205 moves in a direction indicated by arrow Z as a result of the sensor undergoing acceleration, the proportion of the volume of the proof mass 205 (dielectric) in the fringe field 204 changes, causing a change in capacitance value. Therefore, by detecting the change in capacitance value, the acceleration the sensor undergoes is detected.
Moreover, in the above-described acceleration sensor 200 proposed in Non-Patent Document 1, unlike the above-described acceleration sensor 100 having the comb teeth 102a as shown in FIG. 47, there is no need for a DRIE process in the fabrication process. This helps prevent a reduction in production efficiency.
Non-Patent Document 1: the Collection of the Lecture Treatises Presented at the Symposium held by Kansai University Organization for Research and Development of Innovative Science and Technology, Vol. 8th, Pages 153-156 (2004. 01. 10)